A triac is a bidirectional component that comprises two conduction or main terminals A1 and A2, and a gate terminal G. If a potential difference exists between the main terminals A1 and A2, the triac may be turned on by applying an adapted triggering current between the gate terminal and terminal A1 which will be called the reference terminal or main terminal associated with the gate. Once triggered, the triac remains conductive as long as the current flowing through it is above a threshold called “the holding current”, and then automatically turns off when this current falls below the holding current.
A triac may be used to control the power delivered to a load coupled to an alternating current source, for example mains/household electricity. The current source is coupled to a series connection of the triac with the load, and the power delivered to the load is controlled by turning on and turning off the triac.
In some cases, dysfunction of the load may lead to abnormally high currents flowing through the triac, which may cause it to be damaged.
As an example, we consider the case where the load is a compressor of a refrigerator. A starting current with an amplitude of approximately 6 to 12 A is used to start up the motor of the compressor. After between 0.4 and 1 second, the current through the triac generally stabilizes to a nominal level, with an amplitude of approximately 1 A or 2 A. If the motor dysfunctions, notably if the motor jams at start up or while running, the current through the triac is maintained at a high level (6 to 12 A) for an undetermined period. Such a current is likely to damage the triac or the motor if it stays high for too long, for example for more than 10 seconds.
A protection system has been proposed that comprises a device for measuring the current running through a triac (between the main terminals A1 and A2), and means for stopping the triggering current of the triac (gate current), or setting up an alarm signal, when an overcurrent is detected. The interruption of the gate current causes the triac to turn off, and the load ceases to be powered.
A disadvantage of such a protection system is that it comprises components specifically dedicated to the measurement of the current through the triac. Among these components, at least one resistor is provided, serially coupled to the triac. Thus, part of the power supplied by the source is dissipated in the protection system.
Another type of protection system has been proposed that comprises a device for measuring the temperature of the triac. An abnormal temperature rise indicates an overcurrent in the triac. Means are provided for interrupting the triggering current (gate current) when the temperature exceeds a threshold. The triac then turns off, and the load ceases to be powered.
A disadvantage of such a system lies in its relatively high cost. In addition, since the temperature rise is relatively slow, the triac is turned off rather late, and may be damaged before the protection system is activated.
A fuse protection system could also be provided. However, in the above example of a refrigerator, it raises the problem of distinguishing between the normal start up current and the current occurring if the motor jams, the latter current having identical amplitude but running through the triac for a longer time. Another disadvantage of a fuse system is that once the protection system has been triggered, the device is made unusable until the fuse is replaced or reset.
It would be desirable to provide a protection system for a triac that is simpler and more efficient than existing systems.
It would notably be desirable to be able to detect easily and rapidly an overcurrent in a triac.